1. Field of the Invention
The present invention generally relates to a fuel injection control apparatus for an internal combustion engine, and more particularly to a fuel injection control apparatus having a correction means for correcting the injection quantity on the basis of fuel deposited on a passage wall of an intake system of an internal combustion engine as well as evaporated fuel generated in such a manner that the fuel deposited on the passage wall evaporates. More specifically, the present invention is intended to efficiently control the injection quantity immediately after the engine is started.
2. Description of the Related Art
In general, when an internal combustion engine is operating in a steady state, a predetermined quantity of fuel is deposited on a passage wall of an intake system so that an equilibrium condition is maintained. There is a correlation between the quantity of fuel deposited on the passage wall (hereinafter referred to as a deposited fuel quantity and the quantity of intake air or an intake manifold pressure). As the intake air quantity increases (the intake manifold pressure increases), the deposited fuel quantity increases. Hence, when the intake air quantity changes from a small quantity to a large quantity (in other words, when the automobile is accelerated), a predetermined quantity of fuel which is part of the injection quantity is deposited on the intake passage wall so that the engine is changed to the equilibrium state. As a result, the injection quantity which is actually injected into a cylinder decreases by the deposited fuel quantity, and thus a mixture of air and fuel becomes lean. On the other hand, when the intake air quantity changes from a large quantity to a small quantity (in other words, when the automobile is decelerated), the fuel deposited on the passage wall is partially evaporated by an excessive quantity with respect to the deposited fuel quantity obtained in the equilibrium state. As a result, the injection quantity which is actually injected into the cylinder increases by the quantity of the evaporated fuel, and thus the mixture becomes rich.
Japanese Laid-Open Patent Publication No. 63-215848 has proposed an improvement intended to prevent deviations in the air-fuel ratio in transient states. According to this publication, a saturation quantity of fuel deposited on the intake passage wall in the steady state is calculated at predetermined intervals using the intake air quantity having a correlation with the deposited fuel quantity. Then, the difference between a saturation quantity obtained at present and a saturation quantity at the immediately previous time is calculated, and the deposited fuel quantity or the evaporated fuel quantity (a change in the deposited fuel quantity) is predicted. Thereafter, a fuel deposition correction quantity is calculated based on the deposited fuel quantity or the evaporated fuel quantity, and a basic injection quantity is corrected using the fuel deposition correction quantity. It can be seen from the above that the method disclosed in the above publication deems that the deposited fuel quantity obtained in the steady state before a transient state is in the saturated state. When the state of the engine changes to a transient state, a change in the deposited fuel quantity from the saturation quantity of fuel deposited on the intake passage wall in the steady state is calculated. Then, the basic injection quantity is corrected based on the results of the above calculation.
However, the above proposed method has the following disadvantages. When a deposit containing carbon particles is deposited on the intake passage (intake manifold) wall, the surface area of the intake manifold increases. This increases the quantity of fuel to be deposited, and hence it takes a long time to obtain the saturated state. Within a predetermined period after the internal combustion engine is started, fuel injections are carried out only a small number of times. This means that the saturation quantity of fuel in the steady state has not yet been deposited on the intake passage wall within the predetermined period after the engine is started. The above-mentioned proposed method calculates the fuel deposition correction quantity on the assumption that the saturation quantity in the steady state is deposited on the intake passage wall. However, in the above case, a quantity of fuel smaller than the saturation quantity is actually deposited on the intake passage wall. Hence, the mixture is lean, and drivability in the accelerating state within the predetermined period after the engine is started, is degraded.